A process of producing a monolithic ceramic capacitor is generally as follows. First, a dielectric ceramic sheet is prepared. Then, an electrode material for an internal electrode is disposed on the sheet. The dielectric ceramic material, for example, includes BaTiO.sub.3 as the main component. Next, a plurality of the dielectric ceramic sheets are laminated. The laminated sheets are pressed with heating to form a laminated body and the laminated body are fired in air at 1250 to 1350.degree. C. Finally, external electrodes applied on the edge surface of the laminated body to connect to the internal electrodes.
Accordingly, the material of the internal electrode is required to meet to following conditions.
(a) Because the dielectric ceramic and the internal electrodes are simultaneously fired, the internal electrode preferably has a melting point same as or higher than firing temperature of the dielectric ceramic. PA1 (b) The material is not oxidized even in an oxidative high-temperature atmosphere and does not react with the dielectric ceramic.
As the electrodes meeting such conditions, a noble metal or the alloy thereof, such as platinum, gold, palladium, a silver-palladium alloy and the like has been used. Although these electrode materials have excellent characteristics, they are expensive. Thus, the amount of the electrode material cost of the monolithic ceramic capacitor becomes from 30 to 70% and becomes the largest factor increasing the production cost of monolithic ceramic capacitors.
Other materials having high melting points than the noble metals include such base metals as Ni, Fe, Co, W, Mo and the like but these base metals are easily oxidized in a high-temperate oxidative atmosphere, whereby they become unusable as electrodes. Accordingly, to use these base metals as the internal electrodes of a monolithic ceramic capacitor, it is necessary to fire the base metal together with a dielectric ceramic in a neutral or reductive atmosphere. However, when conventional dielectric ceramic materials are fired in such a neutral or reducing atmosphere, they are greatly reduced and become semi-conductive.
To overcome this problem, there are proposed, for example, a dielectric ceramic material wherein the barium site/titanium site ratio is in excess of the stoichiometric ratio in a barium titanate solid solution as shown in JP-B-57-42588 and a dielectric ceramic material made up of a barium titanate solid solution combined with an oxide of a rare earth element such as La, Nd, Sm, Dy, Y, etc., as shown in JP-A-61-101459.
Also, as a dielectric ceramic material having a small temperature dependence of the dielectric constant, there are proposed, for example, a dielectric ceramic material of a BaTiO.sub.3 --CaZro.sub.3 --MnO--MgO system composition as shown in JP-A-62-256422 and a dielectric ceramic material of a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 system composition as shown in JP-B-61-14611.
By using such a dielectric ceramic material as described above, a dielectric ceramic which does not become a semi-conductive material even when the material is fired in a reducing atmosphere and the production of a monolithic ceramic capacitor using a base metal such as nickel and the like as the internal electrodes becomes possible.
With the recent developments in electronics, miniatualizing of electronic parts has quickly proceeded and the tendency to small-sizing and increasing the capacity of monolithic ceramic capacitors also is remarkable. Thus, the increase of the dielectric constant of a dielectric ceramic material and thinning of a dielectric ceramic layer have proceeded very quickly. Accordingly, the demand for a dielectric ceramic material having a high dielectric constant, showing a small temperature change of the dielectric constant, and being excellent in reliability has become large.
However, the dielectric ceramic materials shown in JP-B-57-42588 and JP-A-61-101459 give a large dielectric constant but have the faults that the crystal grains of the dielectric ceramic obtained become large, so that when the thickness of the dielectric ceramic layer in the monolithic ceramic capacitor becomes as thin as 10 .mu.m or thinner, the number of the crystal grains existing in one layer is reduced, and the reliability is lowered. Furthermore, there is also a problem in the dielectric ceramic materials, in that the temperature change of the dielectric constant is large. Thus, the above-described dielectric ceramic materials cannot meet the requirements of the market.
Also, in the dielectric ceramic material shown in JP-A-62-256422, the dielectric constant is relatively high, the crystal grains of the dielectric ceramic obtained are small, and the temperature change of the dielectric constant is small but because CaZrO.sub.3 and also CaTiO.sub.3 formed in the firing process are liable to form a secondary phase with MnO, etc., there is a problem in reliability at a high temperature.
Furthermore, in the dielectric ceramic material shown in JP-B-61-14611, there are faults that the dielectric constant of the dielectric ceramic obtained is from 2,000 to 2,800 and that the material is disadvantageous from the view point of small-sizing and increasing the capacity of the monolithic ceramic capacitor. Also, there is a problem in that the dielectric ceramic material cannot satisfy the X7R characteristics prescribed by the EIA standard, that is, the characteristics that the changing ratio of the electrostatic capacity is within .+-.15% in the temperature range of from -55.degree. C. to +125.degree. C.
Also, various improvements in the anti-reducing dielectric ceramics proposed heretofore have been made on preventing deterioration of the insulating resistance in a high-temperature loading life test but the deterioration of the insulating resistance in a moisture loading test has not been so improved.
Thus, to solve the above-described problems, various components are proposed in JP-A-5-9066, JP-A-5-9067, and JP-A-5-9068. However, because of the requirement for further small-sizing and further increasing the capacity, the requirements of market for thinning the thickness of a dielectric ceramic layer and reliability have become more severe and the requirement for a dielectric ceramic material having better reliability and coping with thinning the layer thickness has increased. Accordingly, the necessary for providing a small-sized and large capacity monolithic ceramic capacitor excellent in the reliability characteristics under high temperature and high humidity has occurred.